The present invention relates to circuit packages for integrated circuits and, more particularly, to such circuit packages that include polymers having high melting temperatures.
Various plastics, such as liquid crystal polymers (LCPs), are used in a wide range of manufactured products, including consumer goods, medical devices and packages for electronic integrated circuits. In many instances, plastic is heated during one or more manufacturing steps of a product or later, such as while the product is in use. For example, many products are manufactured by an injection molding process, which includes heating plastic to soften it and injecting the softened plastic into a mold. The plastic takes on the shape of the mold and (ideally) retains that shape during subsequent manufacturing steps and throughout the useful life of the resulting product.
Some manufacturing processes and products could be improved through the use of a plastic that has a higher melting temperature than that of conventional plastics. Other manufacturing processes and products would benefit from the use of a plastic whose melting temperature can be raised after the plastic has been molded into a shape, but before a subsequent step that involves a temperature higher than the plastic's original melting point.
For example, while operating, many devices in microelectronics circuit packages (such as some integrated circuits used in high-power radio transmitters) dissipate large amounts of heat. In such a circuit package, solder is used to attach a microelectronic device (a “die”) to a metal or other heat-conductive component (such as a mounting “flange” or “leadframe”) to maximize heat transfer from the die to the metal component. In a typical application, the metal component is attached to a heat sink, which is cooled, such as by natural air convection, forced airflow (typically from a fan) or a circulating cooling liquid.
Various solders, including gold-tin (AuSn) and gold-silicon (AuSi), are used to attach the die to the metal component of the circuit package. When the die is attached to the metal component, the solder is heated to a temperature (such as between about 280° C. and about 320° C. for AuSn or between about 390° C. and about 430° C. for AuSi) that exceeds the melting temperatures of conventional plastics. For example, most conventional plastics melt below about 300° C., and most liquid crystal polymers melt at temperatures near 330° C. or slightly higher. Consequently, conventional plastics, including conventional LCPs, cannot be used in circuit packages.
Instead, manufacturers typically use a ceramic material. However, ceramics must be brazed to the other components of a circuit package. Brazing is a high-temperature (approximately 800° C.) process, which creates mechanical problems, such as out-of-flatness, in the circuit packages. Furthermore, manufacturing processes that involve ceramics are expensive.
On the other hand, plastic is relatively inexpensive, and some LCPs have superior dielectric properties and other characteristics that would be desirable in electronic circuit packages. Unfortunately, the relatively low melting points of conventional LCPs and other plastics have prevented their practical use in such packages.